I could be missing something, but I can't really wrap my head around "unlimited paralelism".

What they say is that the logic is embedded into their server binary and they write to a local EBS. But what happens when they have two servers? EBS can't be rw mounted in multiple places.

Won't adding the second and more servers cause trouble like migrating data when new server joins the cluster, or a server leaves the cluster?

I understand Aurora was too expensive for them. But I think it is important to note their whole setup is not HA at all (which may be fine, but the header could be misleading).